1. Field of the Invention
The invention relates to metal film deposition. More particularly, the invention relates to enhancing deposition of a metal film within a feature on a substrate.
2. Description of the Background Art
As circuit densities increase, the widths of features, such as vias, trenches, and electric contacts, as well as the width of the dielectric materials between these features, decrease. However, the height of the dielectric layers remains substantially constant. Therefore, the aspect ratios of the features, i.e., the features height or depth divided by its width, increases. The concurrent reduction of width and increase in aspect ratio of the features poses a challenge to traditional metal film deposition techniques and processes because reliable formation of interconnect features are required to increase circuit density, to permit greater power density endured by interconnect features, and to improve the quality of individually processed substrates.
Electroplating, previously limited in integrated circuit design to the fabrication of lines on circuit boards, is now being used to deposit metal films, such as copper, within features formed in substrates. Electroplating, in general, can be performed using a variety of techniques. One embodiment of an electroplating metal film deposition process involves initially depositing a diffusion barrier layer over the feature surface by a process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). A seed layer is then deposited on the substrate over the diffusion barrier layer by PVD or CVD. Finally, the metal film is deposited on the seed layer by electroplating. The metal film layer can be planarized by a process such as chemical mechanical polishing (CMP) to define conductive interconnect features.
Deposition of the metal film during electroplating is accomplished by providing an electric current between the seed layer on the substrate and a separate anode. Both the anode and the substrate seed layer are immersed in an electrolyte solution containing metal ions that are to be deposited on to the seed layer. The anode also generates metal ions in the electrolyte solution. FIGS. 2A and 2B show cross sectional views of the progression of a deposited metal film, such as copper, within a single feature 202 on substrate 200 that is representative of all of the features formed on the seed layer. FIG. 2A shows a substrate having undergone PVD or CVD processing in which a seed layer 220 has been deposited on all the surfaces of feature 202 including the horizontal field 204, the walls 206, and the bottom 208. FIG. 2B shows the substrate 200 having an electroplated metal film 215 deposited on the seed layer 220. To provide uniform electric characteristics, it is important to deposit a substantially even metal film 215 on those portions of the seed layer 220 that extend over the horizontal field 204. It is also important to deposit a metal film that completely fills the feature 202 without any voids or air gaps in the feature.
As the dimensions of the features decrease below sub-micron dimensions, the dynamics associated with supplying metal ions within the electrolyte solution into the features becomes difficult to control. Due to the small opening (e.g., throat of the feature), one of the technical challenges involves depositing more metal ions into the features through the throat to form the metal film. Ion starvation resulting from the concentration of the metal ions supplied into the features to replace the metal ions that leave deposited as metal film in the features is limited. As such, the concentration of metal ions in the electrolyte solution contained within the features requires rejuvenation. xe2x80x9cIon-starvationxe2x80x9d, as shown in FIG. 2B, often occurs during plating of features having a small dimension (i.e. less than 1 xcexcm) in which insufficient metal ions are supplied to within the feature to limit the concentration of metal ions in the feature. Because of ion-starvation in the feature, the metal film deposition rate at the throat of the channel 212 exceeds the metal film deposition rate on the walls 206 or bottom 208 of the feature, and frequently creates a void 214 within the feature 202. Completely filling the features 202 with metal film is difficult because of the minute size of the features, because the features are oriented at different angles, and because an increased charge density causes more deposition at the edges and corners (i.e. at throat 212) of the features. An overhang of the seed layer 220 also leads to the void 214. The electrical characteristics of features having a void is unpredictable, and parts having features formed with voids are not suitable for use in a reliable electronic device.
It is desirable to use high metal film deposition rates, within the features and in the field surrounding the features, both for higher processing throughput and for increased utilization of the associated processing equipment. The deposition rates are largely a function of the bias voltage applied to the substrate. However, if the initial bias voltage applied to the substrate is too high, there is an increased tendency to choke off the feature at throat 212. Therefore, the initial bias voltage in present electroplating systems is often reduced to approximately 0.8 volts until such times that the features have started to fill.
In a so-called xe2x80x9cbottom-upxe2x80x9d electromagnetic field that is applied through the electrolyte solution between an anode and a seed layer during a bottom-up deposition process, the current density and the associated metal film deposition rate on of the bottom 208 exceeds that on the horizontal field 204 or the walls 206. The goal of bottom-up deposition is to completely fill a feature with metal film yielding a substrate 200 having filled features. After the feature is completely filled, all further metal film deposition will increase the depth of the horizontal field 204.
Such bottom-up deposition processes are difficult to achieve in practice minute size features (in the sub-micron range). During plating in features having small dimensions, it is make it difficult to replace metal ions in the electrolyte solution that are deposited during the plating, to maintain a sufficient metal ion concentration within the electrolyte solution in the feature. As the metal ions are deposited on the surfaces of the features as metal film, the concentration of metal ions remaining in the electrolyte solution within the feature decreases. Maintaining the concentration of metal ions in the electrolyte solution within the feature is therefore important during the metal deposition process to provide the desired deposition rate of metal film within the features.
One technique for minimizing deposits that close off a throat 212 before the remainder if the feature is filled is to apply an alternative series of deposition and etch steps, i.e. dep-etch steps. Each deposit portion of the cycle deposits metal ions from the electrolyte solution into the features 202 and on to the horizontal field 204, while, unfortunately, also creating buildup at throat 212. Each etch cycle then partially etches the metal film in the horizontal field 204 on the substrate to keep the throat open. The deposits forming on the wall 206 and the bottom 208 are etched at a lower rate, during the etch cycle, than those on the horizontal field 204 as a result of the minute size of the features. However, the dep-etch technique is time consuming and substantially reduces throughput of substrates.
Therefore, there remains a need for an ECP system that enhances the concentration of metal ions contained in the electrolyte solution within the features and increases the deposition rate within those features, resulting in improved overall processing throughput.
In one aspect, a method and associated apparatus of electroplating an object that has small features is provided. The method comprises immersing the plating surface into an electrolyte solution and mechanically enhancing the concentration of metal ions in the electrolyte solution contained in the features. In one embodiment, the mechanical enhancement comprises mechanically vibrating the plating surface. In another embodiment, the mechanical enhancement comprises mechanically vibrating the electrolyte solution. In a further embodiment, the mechanical enhancement comprises increasing the pressure applied to the electrolyte solution.